The invention concerns an automatic focusing system for a photographic camera (a so-called autofocus system), with a memory unit to electronically store distance values and a linkage or computing circuit to ascertain the most advantageous focus of the objective resulting from the measured values, taking into account various requirements of location and depth of field.
Known automatic focusing systems measure the distance to an object within a very limited range and at the center of the image field to be photographed.
This procedure offers good results only when the main object also is located at the image center or when the object as a whole assumes only a slight depth.
This procedure is improved by introducing a memory for the measured values. Thus first aim may be taken of the main object by means of a target, its distance may be measured and the distance so obtained then may be stored in a memory. After that the camera can be pivoted into another direction to achieve the desired image composition. As the shutter is released, the focusing of the objective then takes place according to the distance data contained in the memory and as a result sharp imaging of the previously measured main object is obtained.
In another suggestion according to the West German published application No. 33 44 431 A 1, distance measurements are carried out at several locations within the image field to be photographed. The measuring field of the distance-measuring or ranging device automatically scans the object to be taken. The least object distance is stored and the object to be photographed is focused for that value. This assumes that the main object always shall be nearest the camera.
All these procedures incur the drawback that the camera focusing always is set only to the particular measured object plane. However, as most objects range in depth, it is hardly possible to deliberately sharply image a specific range of depth of the object, even though this is possible in view of the ever present depth of field of the objective. Therefore the depth of field, which is present especially for the smaller stops of the objective, cannot be fully utilized.